Two zero lines
Ordnance Datum Malin (OD Malin) is the zero for land. It is mean sea level at Malin Head measured through the 1960s, and every height on an Irish Ordnance Survey map is measured from it. The Marine Institute reports its tide gauges and predictions against OD Malin, and so does TideTimes.ie. Because it is close to average sea level, high waters come out positive and low waters negative.
Chart datum is the zero for the sea. Hydrographers set it at or near Lowest Astronomical Tide (LAT), the lowest level the moon and sun can produce. Depths on a nautical chart and heights in a harbour tide table are measured from it. Almost every tide is above it, so harbour tables show only positive numbers, and the depth on the chart is the least water you would expect in normal weather.
Same water, two rulers. A Dublin low water of −1.0 m on our page is about +1.4 m in the Dublin Port tide table.
The offset at Irish gauges
The Marine Institute publishes each tide gauge’s level against both datums. The difference is how far LAT sits below OD Malin at that gauge. Rounded to 0.1 m:
| Gauge | LAT below OD Malin |
|---|---|
| Dublin Port | 2.5 m |
| Howth | 2.6 m |
| Skerries | 2.9 m |
| Wexford Harbour | 1.0 m |
| Rosslare | 1.1 m |
| Dunmore East | 2.5 m |
| Ballycotton | 2.5 m |
| Union Hall | 2.1 m |
| Castletownbere | 2.1 m |
| Dingle | 2.4 m |
| Kilrush | 3.0 m |
| Galway | 3.0 m |
| Inishmore | 2.8 m |
| Ballyglass | 2.1 m |
| Sligo | 2.3 m |
| Killybegs | 2.3 m |
| Aranmore | 2.2 m |
| Malin Head | 2.1 m |
| Buncrana | 2.3 m |
To turn one of our heights into a height above LAT, add the number in the table. The pattern follows the size of the tide. Where the range is big (Galway, the Shannon estuary at Kilrush) the lowest possible tide is a long way below average sea level. Where it is small (Wexford and Rosslare, under 2 m on a big spring) LAT is only about a metre down.
Chart datum is not always exactly LAT
Older harbour datums were fixed before LAT was computed and were kept so that decades of records stayed comparable. Dublin is the clearest case. Dublin Port Company’s tables state that their chart datum is 0.20 m above Ordnance Datum (Poolbeg), the old Dublin land datum, and list Lowest Astronomical Tide as 0.10 m below chart datum. In practice: add about 2.4 m to our Dublin heights to match the Dublin Port table.
We checked this on the predictions for 1 November 2026. Our page gives the afternoon high at 16:20, 1.40 m; the Dublin Port table gives 16:10, 3.78 m. The morning low is 09:20, −1.01 m against 09:24, 1.43 m. Height differences are 2.38 m and 2.44 m. The few minutes between the times come from the different gauge positions (the North Wall and the Marine Institute gauge are at different spots in the port) and the different prediction methods.
When the datum matters to you
- Launching a boat. Pilot books and harbour notes often say a slip “dries at 1.0 m” or has “2 m at MHWS”. Those are chart datum numbers. Convert our height before you compare.
- Reading a chart. A charted depth of 0.5 m with our predicted low of −1.0 m at Dublin means about 0.5 + (−1.0 + 2.4) = 1.9 m of water over that spot at low tide, before weather.
- Flood and property maps. Planning documents, flood maps and survey drawings in the Republic use OD Malin, the same zero as our pages. A garden wall at 2.5 m OD Malin in Dublin is 0.35 m above the biggest spring high water we list this winter (2.15 m), before any surge.
- Comparing sites. Comparing a TideTimes.ie page with another site? Check which datum the other site uses. If its Dublin numbers run from about 0.5 m to 4 m, it is on chart datum.
Never navigate from this or any website alone. Use current Admiralty or Irish charts and the harbour’s own notices. For days out, the tide table guide covers the rest.